Muffler for air compressor

ABSTRACT

For the combined intake section of a compressed-air generating system comprising an air compressor and an air dryer, the use of an additional muffler is proposed for an air-spring system that is employed mainly in passenger vehicles. This muffler is installed directly in a rubber hose leading to the atmosphere. Both air for the air compressor is sucked in and air for regeneration of the air dryer is discharged to the atmosphere through this muffler. The muffler has a simple construction, because of the fact that a damping insert, composed of a roll of noise-reducing knitted fabric, is pushed into the rubber hose, and throttling of the air stream is additionally provided at the point of the port to the atmosphere.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to a muffler and, in particular, to a muffler for an air-spring system of the type used in passenger vehicles.

[0002] Mufflers are used to reduce the noise output from air compressors. FIG. 2 shows a conventional muffler, designed for a passenger car, for the intake section (1) of a combined functional unit comprising air compressor (15) and air dryer (16), together with an air inlet (2) on the air compressor and an air outlet (3) that leads to the air dryer, while functional units that form the air-inlet path are disposed on the air inlet and functional units that form the air-outlet path are disposed on the air outlet.

[0003] The functional units of the air-inlet and air-outlet paths form intake section (1), which is designed as a combined intake section as air-inlet path units are also used for the air-outlet path.

[0004] Besides the air inlet and the air outlet, the combination of air compressor and air dryer is provided with a pressure port (26), by which an air-spring system installed in the passenger car is supplied with air.

[0005] In the combined intake section (1) according to FIG. 2, there is provided on air inlet (2) a first tubular air line (4), which is connected to an air-line tee (10), which on the outlet side is connected to the outlet (9) of an air filter (7), whose inlet (8) is connected to a third tubular air line (6), which ultimately provides the connection to the atmosphere (11); these functional units represent the air-inlet path.

[0006] Air outlet (3) is connected via a second tubular air line (5) to the branch of the air-line tee (10), so that air discharged by the air outlet flows through air filter (7) and third tubular air line (6) to the atmosphere (11). Together these functional units form the air-outlet path.

[0007] Air filter (7) is composed of a pleated unit of air-permeable paper filter elements, which have the ability to retain dirt particles from the air flowing from the atmosphere (11) to air inlet (2), thus effecting air cleaning.

[0008] In a combination of this known type comprising air compressor (15) and air dryer (16), both intake of air at the air inlet (2) by the air compressor (15) and discharge of air at the air outlet (3) by the air dryer (16) take place at different times; thus air from the air outlet (3) and air sucked in at the air inlet (2) pass through the air filter in different directions, with the effect of self-cleaning of the air filter. The dirt particles retained during air-inlet flow in the air filter are discharged to the atmosphere once again by the air-outlet air.

[0009] In addition to this function as a self-cleaning filter, the air filter also muffles the air noises.

[0010] Air noises, which are produced during intake and especially during venting (venting blast), are very disturbing to the occupants of the passenger car and greatly impair their comfort.

[0011] In the prior art, noise damping by about 3 dB(A) has been achieved in the described manner, but this degree of reduction cannot be regarded as entirely satisfactory. Especially in higher-priced passenger cars, in which air-spring systems are preferably used and which are also equipped, for example, with a high-performance audio system, every possibility of suppressing additional disturbing noises created by the compressed-air-generating combination of air compressor and air dryer must be exploited in order to improve the comfort of the occupants.

[0012] Accordingly, it is desired to improve the muffling of a combined intake section of an air compressor/air dryer to further reduce noise emission.

SUMMARY OF THE INVENTION

[0013] Generally speaking, in accordance with the present invention, a muffler is provided for reducing the noise emission of a combined intake section of an air compressor/air dryer that improves over prior art mufflers.

[0014] In one preferred embodiment of the present invention, a muffler is provided for use in an air-consuming system comprising a compressed-air generating device including an air compressor, an electric motor for powering the air compressor and an air dryer, a pressure port for pressurizing and venting the air consuming system, an air inlet for sucking air from the atmosphere and an air outlet for discharging air-dryer regenerating air to the atmosphere. An air line connects the air filter to the atmosphere. The muffler is constructed from an air-permeable, sound-absorbing, meshed fabric of thermoplastic threads and is formed as the air line.

[0015] In another embodiment of the present invention, a multi-functional muffling unit is provided for use in the above described air-consuming system. The multi-functional muffling unit includes an air filtering element and a muffling element.

[0016] In yet another embodiment of the present invention, the muffler or multi-functional muffling unit is a compact preassembled unit that is easily coupled to the air-consuming system by use of slip-on joints and clamp joints.

[0017] In yet another preferred embodiment of the present invention, the air-lines of one of the previously described air-consuming systems are formed by mandrel-sleeve-joint technique.

[0018] Accordingly, it is an object of the present invention to provide improved noise emission reduction in vehicles employing an air compressor/air dryer with a combined intake section.

[0019] Another object of the present invention is to provide a muffler that can be manufactured for very low cost while achieving good noise-damping properties.

[0020] A further object of the present invention is to provide a complete intake section that is constructed as a preassembled, compact component, which can be easily connected to the unit comprising an air compressor and an air dryer by simple slip-on and clamp joints.

[0021] Another object of the present invention is to provide a muffler wherein the major part of the air lines between the elements of the intake section can be manufactured with mandrel-sleeve joints, which are particularly cost-effective.

[0022] Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.

[0023] The present invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings in which:

[0025]FIG. 1 is a schematic diagram of the combined intake section together with the compressed-air generating system comprising an air compressor and an air dryer according to the present invention;

[0026]FIG. 2 is a schematic diagram of a combined intake section according to the prior art;

[0027]FIG. 3 is a perspective view of the inventive combined intake section;

[0028]FIGS. 4a-c depict various views of the inventive muffler with a noise-damping insert disposed in a connecting hose;

[0029]FIGS. 5a-c illustrate how the noise-damping insert is prepared for use; and

[0030]FIGS. 6a-b show an embodiment of the invention in which the noise-damping insert is disposed directly in the air filter itself.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0031] Reference is made to the drawing figures where like reference numerals are used for corresponding parts.

[0032] As shown in FIG. 1, compressed-air generating system (23) comprises an air compressor (15) powered by an electric motor (14) and operated together with an air dryer (16), such as type 899 381 29 2 of WABCO GmbH & Co. OHG, assignee of the present application. The system (23) is intended for use in pneumatic systems, especially for an air-spring system, for example in a passenger car.

[0033] To inflate the air-spring bellows of the air-spring system for the purpose of raising the vehicle chassis above the vehicle axles, electric motor (14) is made to rotate via an electric or electronic control unit (not illustrated), and air compressor (15) delivers air from air inlet (2) via the dryer (16) and first check valve (24) to pressure port (26) for the “consumer” of the air-spring system, which strictly speaking does not consume air but injects it into the air-spring bellows in the manner required by the nature of the closed air-spring system.

[0034] In this process, the compressed air generated during this air-compressor delivery phase is dried in the cartridge of air dryer (16), in order to avoid icing of valves that may be needed in connection with the air-spring system. During such drying in the delivery phase, the granules of the air-dryer cartridge absorb the moisture contained in the delivered air.

[0035] To lower the level of the vehicle chassis, the quantity of air in the air-spring bellows is reduced, and this quantity of air is simultaneously used for drying the cartridge granules. For this purpose, venting solenoid valve (27) is operated by the control unit, whereupon pressure builds up at the pneumatic switching inlet of venting relay valve (28), thus overcoming the force of the restoring spring and switching the valve to passing condition.

[0036] Venting from pressure port (26) takes place via the drying path, opened venting solenoid valve (27), throttle (29), second check valve (25), which is opened counter to its closing direction, air dryer (16) and venting relay valve (28), which is switched to passing condition, to air-dryer air outlet (3). Because of the effect of throttle (29), the drying air stream through air dryer (16) is greatly reduced, thus ensuring that regeneration of the air-dryer cartridge takes place efficiently, while the large air cross section of opened venting relay valve (28) ensures that the regeneration air is passed to air outlet (3) without buildup of dynamic pressure.

[0037] In addition, the operating point of venting relay valve (28) can be set to various values via the indicated variable restoring setting of the spring. Below this operating point, venting relay valve (28) returns to its blocking state, thus preventing the air-spring bellows from being completely vented and destroyed by wrinkling; in this way a residual-pressure holding function is achieved. Furthermore, a safety-valve function is obtained via the pneumatic pilot device, which is shown as a broken line extending from air dryer (16). This ensures that venting relay valve (28) will open and thus prevent damage to the system if an air-pressure safety threshold, for example of 16 bar, is exceeded during delivery of air to air dryer (16).

[0038] All of the compressed air delivered via pressure port (26) into the air-spring system by reinjection via pressure port (26) is reused for regeneration of air dryer (16); in this way it is ensured that sufficiently dried air will be ready at the appropriate time for the air-spring system. In contrast, air-spring systems that lower the level by releasing air from the air-spring bellows via valves directly into the atmosphere, without passing this air via air dryer (16), do not offer sufficient safety for adequate air drying, making use of a compressed-air generating system (23) of the type explained hereinabove.

[0039] A combined intake section (1) is connected to air inlet (2) and air outlet (3) of compressed-air generating system (23). The term “combined” means that a first air line (4) leading to air inlet (2) is connected via trunk bore (30) and branch bore (31) of an air-line tee (10) to a second air line (5) leading to air outlet (3). Trunk bore (30) of the air-line tee (10) leads further to outlet (9) of air filter (7). Thus, both the intake air to air inlet (2) and the venting air discharged by air outlet (3) flow through the air filter, thus cleaning the air compressed in air compressor (15) and ensuring that the micropores of air filter (7) are not permanently fouled by the dirt particles from the intake air.

[0040] At inlet (8) of air filter (7) there is provided a third air line (6), which connects air-filter inlet (8) to the atmosphere (11).

[0041] As explained, combined intake section (1) is therefore connected to compressed-air supply system (23), and it connects the system to the atmosphere (11); this coupling, however, also results in acoustic coupling between the atmosphere and surroundings and thus between the occupants of the vehicle and compressed-air supply system (23).

[0042] Compressed-air supply system (23) produces two kinds of noises: air-compressor running noises and air-dryer air noises. The running noises of the air compressor occur mainly during air delivery, in the form of running noises in the motor bearing, in the connecting-rod bearing and in the region of the piston ring of the air-compressor delivery piston; a particularly important contribution to these noises results from the point of intake from the atmosphere (11) being directly connected acoustically to the crankcase of air compressor (15).

[0043] The air noises caused during air drying include a whistling sound, which occurs during air-dryer regeneration as the air, throttled via throttle (29), escapes from the air-spring bellows to the atmosphere (11). The main problem in addition to these whistling noises is a very disturbing venting blast, which occurs at the beginning of air-dryer regeneration.

[0044] During the air-compressor delivery phase, the delivery pressure of air compressor (15) is present directly at the inlet of air dryer (16). This can have a value of 16 bar, for example, and so the volume of the air-dryer cartridge is filled with 16-bar air. Because of the well-sealed effect of the overall system, this high air pressure persists even in the time following delivery by the air compressor. In many cases, the air-dryer cartridge volume is still under relatively high pressure even at the beginning of changeover of relay valve (28) for initiation of the air-dryer regeneration process.

[0045] When venting relay valve (28) opens, the air-dryer cartridge, which is under high pressure, is therefore suddenly depressurized via the large free cross section of opened venting relay valve (28), air outlet (3) and combined intake section (1) leading to the atmosphere (11), thus producing the aforesaid highly audible venting blast. The noise-damping measures described hereinafter are intended in particular to damp these highly audible regeneration venting blasts appropriately, as well as the running noises and venting whistles described hereinabove.

[0046] According to the present invention, the third air line (6) is itself designed as an acoustic damping device or muffler, by the fact that a sound-absorbing insulating material (12) is installed in this line itself; this is indicated in FIG. 1 with the circuit-diagram symbol for a muffler.

[0047] Whereas air lines (4, 5, 10) in the combined intake section between air filter (7) and ports (2, 3) have a single diameter (or, in other words, nominal width, which can be 4 mm, for example, but is matched to the air capacity of compressed-air generating system (23)), third air line (6) preferably has a larger diameter such as, for example, 10 mm. Whereas air lines (4, 5, 10) can also be designed as tubular lines, for example made of plastic, third air line (6) is intentionally designed as a flexible rubber hose; this, in its own right, already contributes to noise reduction, because the air molecules in the air stream are not reflected at, for example, the surface of a plastic tube, which can be even and hard, but impinge on the uneven and elastically compliant inside surface of the hose, losing their energy by interaction. In FIG. 3, the manner in which sound-absorbing insulating material (12) is installed in third air line (6) is illustrated by means of a partial section through third air line (6); it has the form of a roll of knitted fabric, as shown in FIG. 5.

[0048]FIGS. 5a-c illustrate how this knitted-fabric roll (12) is prepared. The basic material is, desirably, a hose-like gauze produced on a circular knitting machine for thermoplastic materials. In other words, it is desirably a knitted fabric comprising thermoplastic threads looped together with one another. The threads have a rectangular cross section of, for example, 0.02×0.08 mm, and the looping process produces meshes in a density of, for example, 30 meshes per 100 mm of length. As illustrated in FIG. 5a, this knitted-fabric hose is trimmed to a length such as 90 mm in a first working step, to produce a hose portion (17).

[0049] In a second working step, hose portion (17) is heat-sealed along its cut surfaces (heat-sealed seams (32)), thus forming a rectangular knitted-fabric insert (18) with a thickness of 2 to 2.5 mm (FIG. 5b).

[0050] Knitted-fabric insert (18) is then rolled up around one of its long edges to form a knitted-fabric roll (12) (FIG. 5c). This knitted-fabric roll (12) is finally pushed into third air line (6). FIGS. 4a-c show pushed-in roll (12) in longitudinal section (A-A) and cross section (C-C). The pushing-in process is simplified by the elastic nature of the third air line (6); the connecting hose is expanded by a suitable spreading tool, and knitted-fabric roll (12) is then inserted into expanded third air line (6) by means of a further tool.

[0051] By analogy to conventional mufflers containing perforated plates, in which cross-sectional changes have a noise-damping effect, knitted-fabric roll (12) installed in third air line (6) achieves noise damping in the invention by the fact that the air continuously experiences cross-sectional changes while flowing through line (6), whereby friction effects leading to noise damping are produced.

[0052] Besides the insertion of knitted-fabric roll (12) into third air line (6), yet another noise-reducing measure is provided: As best shown in FIG. 4b, third air line (6) is narrowed (e.g., to a cross section of 6 mm) at its outlet point to the atmosphere (11), and so a throttle (13) is formed there. This throttle causes development of a slight dynamic pressure, thus reducing the flow velocity in third air line (6) and in this way having an additional noise-reducing effect.

[0053] The following table provides a representative summary of the acoustic effect of all of the explained noise-damping measures, as determined on a model of the inventive combined intake section (1) in combination with the compressed-air generating system (23): Individual measure Relates to Improvement Combined intake section, lines assembled  0 dB(A) without further measures 1. Integration of air filter (7) Prior art  3 dB(A) 2. Knitted-fabric roll (12) in third air line (6) 13 dB(A) 3. Throttle (13) in third air line (6)  3 dB(A) 4. Total of all measures 19 dB(A)

[0054] Compared with a combined intake section without any noise-damping measure, connection of air filter (7), as in prior art arrangements, achieves noise damping of about 3 dB (A). Compared with this prior art, knitted-fabric roll (12) in third air line (6) adds further noise damping of 13 dB (A), and throttle (13) then reduces the noise level by a further 3 dB (A). Compared with the prior art, therefore, noise damping totaling 16 dB (A) is achieved with the invention, while all damping measures together result in total noise damping of 19 dB (A).

[0055] It is noted that the explained mesh size of knitted-fabric roll (12) is very large compared with the pore size of paper ultra-fine filter (7); thereby the self-cleaning—explained hereinabove—of air filter (7) by the dryer regeneration air discharged at air outlet (3) is not impaired.

[0056]FIG. 3 shows that the combined intake section (1) can be constructed as a compact functional unit, equipped with slip-on pneumatic fittings, a fitting (19), coupled with first air line (4), for insertion into port (2) of the air inlet, and a fitting (20), coupled with the second air line (5), for insertion into port (3) of the air outlet. These insertion processes at the corresponding ports are indicated by dashed arrows (33 and 34) in FIG. 3.

[0057] In third air line (6), in the region of the outlet to the atmosphere, there is molded on a rubber double bead (35), which can be inserted very simply into a groove-like recess in order to fasten third air line (6). With these features, or in other words fittings (19) and (20) as well as rubber double bead (35), a completely preassembled intake section can be installed by being slipped onto or clamped onto compressed-air generating system (23), which is also preassembled.

[0058]FIG. 3 also shows a particularly cost-effective assembly of the lines and elements of inventive combined intake section (1). Fitting (19) is connected to first air line (4), which in turn is connected to branch (31) of tee (10) in the form of a pneumatic sleeve joint; in the same way, air filter (7) is connected via its outlet (9) to trunk port (30) of tee (10), this trunk port is connected to second air line (5), and this line is connected to fitting (20).

[0059] In the mandrel-sleeve-joint technique, a tubular air line is simultaneously used as a fastening means, by the fact that it is pushed with a tool over the element to be coupled, thus forming a pressure-tight joint. To characterize the mandrel-sleeve-joint technique, the respective enveloping parts of a joint are shown as “corrugated” elements (36) in FIG. 3. The mandrel-sleeve-joint technique represents a very inexpensive connection method and it is desirable to use the technique whenever possible, for example at the coupling of fitting (19) with first air line (4). It should be understood that the coupling to third air line (6) is not amenable to this technique. Desirably, a hose clamp (37) is used instead to secure the enveloping part of third air line (6).

[0060] Another configuration of the invention is illustrated in FIGS. 6a,b. The sound-absorbing insulating material (12) is repositioned from third air line (6) to air filter (7), so that this is formed as a combined functional unit (21) comprising a filter and a muffler. Air filter (7) is illustrated in FIG. 6a, and functional unit (21) comprising a filter and a muffler is illustrated in FIG. 6b.

[0061] Air filter (7) has the form of a cylindrical hollow body provided with inlet (8) and outlet (9), (explained hereinabove) which are molded onto a lower housing part (38) and upper housing part (39), which were initially separate parts (38, 39). For air filtering there is provided a first insert cartridge (40), which is made of air-permeable paper filter elements. For installation of air filter (7), first insert cartridge (40) of the air filter is placed in the cavity of one of the housing parts (for example, (38)), after which the respective other housing part ((39) in the example) is slipped on over installed first insert cartridge (40) and the two housing parts (38, 39) are heat-sealed to one another by an airtight seam (49).

[0062] In functional unit (21) comprising a filter and a muffler as shown in FIG. 6b, one of the two housing parts (38, 39), which can have the same construction, has sufficiently enlarged overall length—in the example of FIG. 6b, this is upper housing part (39)—that a second cartridge (41), made of the explained air-permeable and sound-absorbing insulating material (12), can be provided in the cylindrical interior space of functional unit (21), in addition to cartridge (40) which, as the first cartridge, is disposed directly at outlet (9) of functional unit (21). The sound-absorbing insulating material (12) of second cartridge (41) is again desirably made of a meshed fabric of thermoplastic threads connected to one another, and is shaped in the form of a knitted-fabric roll; this knitted-fabric roll (12) is no longer an elongated cylindrical body with small diameter, but instead is a short cylindrical body with large diameter. Installation takes place by inserting first and second cartridges (40, 41) into empty housing parts (38, 39) and joining two housing parts (38, 39) by an airtight heat seal (49). In this embodiment also, the sound-absorbing insulating material (12) is disposed between first cartridge (40) and the connection to the atmosphere (11) of third air line (6), which is connected in the manner explained hereinabove to inlet (8) of functional unit (21).

[0063] In the configuration of inventive combined intake section (1) using a filter unit (21) comprising a filter and a muffler as illustrated in FIG. 6b, third air line (6) is empty. Nevertheless, for the reasons explained hereinabove, it is provided with throttle (13) at the connection to atmosphere (11) in this configuration also. The basic construction of combined intake section (1) therefore continues to be similar to the configuration illustrated in FIG. 3.

[0064] As an example, first cartridge (40), which is inserted in the air filter, has a diameter of 40 mm and a length of 60 mm, whereas second cartridge (41), which is inserted in the muffler, has the same diameter but a length, for example, of 30 mm. By virtue of these dimensions—which are larger than in the embodiment according to FIG. 4—for second cartridge (41) of damping material, second cartridge (41) can be made somewhat more simply. In addition, insertion of this second cartridge (41) into housing (38, 39) is simplified as compared with the process of pushing elongated knitted-fabric roll (12) into third air line (6), which is expanded by a tool. Thus the manufacturing costs for this embodiment are lower than that for the embodiment according to FIG. 3. A further advantage is that more room for sound-absorbing insulating material (12) is available in housing (38, 39) of functional unit (21) as such than is the case in third air line (6) for sound-absorbing insulating material (12) according to FIGS. 4a-c. Thus it may be possible to achieve a further improvement of noise damping by increasing the mass of damping material.

[0065] Accordingly, the present invention provides a muffler that can be manufactured at a low cost while achieving improved noise emission reduction over prior art mufflers. Embodiments of the present invention may be constructed as a preassembled compact component, which can be easily connected to an air spring system.

[0066] It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above constructions without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

[0067] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween. 

What is claimed is:
 1. In an air-consuming system, said air-consuming system having a compressed-air generating device including an air compressor, an electric motor for powering said air compressor and an air dryer, a pressure port for at least one of pressurizing and venting said air-consuming system, an air inlet for sucking air from the atmosphere, said air inlet in communication with said air compressor and an intake device, an air outlet for discharge of air-dryer regeneration air to atmosphere, said air outlet in communication with said air dryer and a venting device, said intake device including a first air line in communication with an outlet of an air filter, a third air line extending from an inlet of said air filter to atmosphere for passing air sucked from atmosphere through said air filter to said outlet of said air-filter and to said air inlet of said air compressor, said venting device including a second air line leading from said outlet associated with said air dryer to said outlet of said air filter and said third air line, and a pneumatic coupling device disposed proximate to said outlet of said air filter pneumatically connecting said intake device and said venting device, a muffler formed as said third air line constructed and arranged to damp sound emissions from said compressed-air generating device.
 2. The muffler of claim 1, wherein said muffler is a compact preassembled unit, and is coupled to said air-consuming system by at least one of slip-on joints and clamp joints.
 3. The muffler of claim 1, wherein at least one of said first air-line, said second air-line, and said third air-line is formed by mandrel-sleeve-joint technique with a different one of said first air-line, said second air-line, and said third air-line.
 4. The muffler of claim 1, wherein said third air-line is an elastic hose connection including an air-permeable, sound-absorbing insulating material.
 5. The muffler of claim 4, wherein said third air line includes a throttle device adapted to narrow said third air-line in the region between said sound-absorbing insulating material and outlet to atmosphere.
 6. The muffler of claim 4, wherein said sound-absorbing insulating material is formed from a meshed fabric of interconnected thermoplastic threads.
 7. The muffler of claim 6, wherein said sound-absorbing insulating material is shaped as a knitted-fabric roll.
 8. In an air-consuming system, said air-consuming system having a compressed-air generating device including an air compressor, an electric motor for powering said air compressor and an air dryer, a pressure port for at least one of pressurizing and venting said air-consuming system, an air inlet for sucking air from the atmosphere, said air inlet in communication with said air compressor and an intake device, an air outlet for discharge of air-dryer regeneration air to atmosphere, said air outlet in communication with said air dryer and a venting device, said intake device including a first air line in communication with an outlet of an air filter, a third air line extending from an inlet of said air filter to atmosphere for passing air sucked from atmosphere through said air filter to said outlet of said air-filter and to said air inlet of said air compressor, said venting device including a second air line leading from said outlet associated with said air dryer to said outlet of said air filter and said third air line, and a pneumatic coupling device disposed proximate to said outlet of said air filter pneumatically connecting said intake device and said venting device, a multi-functional muffling unit including said air filter and a muffler constructed and arranged to damp sound emissions from said compressed-air generating device.
 9. The multi-functional muffling unit of claim 8, wherein said multi-functional muffling unit is a compact preassembled unit, and is coupled to said air-consuming system by at least one of slip-on joints and clamp joints.
 10. The multi-functional muffling unit of claim 8, wherein at least one said first air-line, said second air-line, and said third air-line is formed by mandrel-sleeve-joint technique with a different one of said first air-line, said second air-line, and said third air-line.
 11. The multi-functional muffling unit of claim 8, wherein said third air line includes a throttle device adapted to narrow said third air-line in the region between said multi-functional unit and outlet to atmosphere.
 12. The multi-functional muffling unit of claim 8, wherein said air filter includes an air-permeable paper filter element and said muffler includes an air-permeable, sound-absorbing insulating material.
 13. The multi-functional muffling unit of claim 12, wherein said sound-absorbing insulating material is formed from a meshed fabric of interconnected thermoplastic threads.
 14. The multi-functional muffling unit of claim 13, wherein said sound-absorbing insulating material is shaped as a knitted-fabric roll. 